In recent years, there has been considerable progress on fabricating microfluidic devices with multiple functionalities, with the goal of attaining lab-on-a-chip [1-3] integration. These efforts have benefited from the development of micro-fabrication technologies such as soft lithography [4]. Polydimethylsiloxane (PDMS) has played an important role for building micro-structures owing to its properties such as transparency, bio-compatibility, and good flexibility [5]. Some complicated micro-devices can be realized by using simple manufacturing techniques such as micro molding with PDMS materials (U.S. Pat. Nos. 7,125,510; 6,692,680; and 6,679,471). However, PDMS is a nonconducting polymer, and patterning metallic structures is very difficult due to the weak adhesion between metal and PDMS. Hence the integration of conducting structures into PDMS has been a critical issue, especially for those applications such as electrokinetic micro-pumps, micro sensors, micro heaters, ER actuators etc. [6-7] that require electrodes for control and signal detection.
Gawron et al. [8] first reported the embedding of thin carbon fibers into PDMS-based microchips for capillary electrophoresis detection. Lee et al. [9] reported the transfer and subsequent embedding of thin films of gold patterns into PDMS via adhesion chemistries mediated by a silane coupling agent. Lim et al. [10] developed a method of transferring and stacking metal layers onto a PDMS substrate by using serial and selective etching techniques. As shown in the U.S. Pat. No. 6,323,659, the electrodes comprising a base material and filler material was disclosed to be used to determine the presence of water in a material. Where a conductive electrode may be formed by depositing carbon black on the elastomer surface, that is accomplished either by wiping on the dry powder or by exposing the elastomer to a suspension of carbon black in a solvent. Alternatively, the electrode may be formed by constructing the entire layer out of an elastomer doped with conductive material (i.e. carbon black or finely divided metal particles). However, incompatibility between PDMS and metal usually causes failures in the fabrication process, especially in the bonding of two materials. Therefore, selection of a right composite with good conductivity, reliable mechanical property, as well as desired thermal characteristics for constructing micro-devices is of great urgency. In particular, the construction of the micro-devices with three-dimensional conducting structures, such as three-dimensional wiring and packaging, represents challenges for the micro-fabrication processing. PDMS-based conducting composites may be promising materials for micro-device fabrication.